The invention relates to a method and a circuit for driving semiconductor power switches, such as Bipolar Junction Transistor (BJT), Metal Oxide Silicium Field Effect Transistor (MOSFET), Silicium Carbide Field Effect Transistor (SiC FET), Galium Nitride Field Effect Transistor (GaN FET) or Junction Field Effect Transistor (JFET) transistors connected in a half bridge HB configuration, which are mainly used as a switch unit to convert power from one form to another in electric motor and generator controlled systems, uninterrupted and interrupted power supplies, voltage DC transmission and in many other conversion applications. Half bridge connection of the power switches controlled with pulse width modulation (PWM) or pulse density modulation (PDM) is a commonly used technique for these applications.
In most conversion applications the half bridge connected power switches switch between both poles of the applied voltage. By alternating the ratio of the conducting time of both power switches and simultaneously setting the duty cycle, the generated output voltage value is defined. The advantage of such a power conversion enables the bidirectional conversion between DC-DC, DC-AC or AC-AC voltage levels theoretically without losses.
Both power switches alternate among two states, namely an on-state and off-state. During the off-state the resistance of the power switch is very high, therefore no load current can flow through it and further no losses are generated, while during the on-state the power switch conducts the load current and the conductive losses are generated due to the internal on-resistance. The value of the internal on-resistance is low enough for the conductive losses to be very low. Changing the states of the individual power switch causes very high losses due to the simultaneity of the operating voltage and the entire load current. Speeding up the state changes reduces the so called switching losses. For the transient time duration the turning-on power switches to over take the entire load current, which in case of the so called hard switch includes also the body diode charge of the second power switch. Accordingly, during a hard switch the switching losses and electro-magnetic interferences EMI increase drastically.
Only one of the two in half bridge connected power switches may be switched on at the same time, since the cross conduction current between the power supply terminals over the power switches would cause a permanent damage to the power switches. In order to prevent any possibility that such a situation can occur the command to turn-on off the second power switch must be performed with an additional minor delay, so-called dead time DT, after the command to turn-off the first power switch. The minimum value of the dead time DT is given by a worst case combination of the driver signal propagation delay and transition time, as well as propagation delay and rise time of the power switches under all possible working conditions, such that a certain time delay, namely interlock delay ID, exists between the executed turn-off of the first power switch and the beginning of the turning-on of the second power switch. The voltage switch between both power switches can be done either by a load inductivity, which constitutes the so called soft switching or by one of the power switches, which constitutes the so called hard switching. The switching type, whether it is a soft or hard switching depends on the load current direction.
Increasing the dead time DT also increases the reliability of the circuit against cross conduction currents, however a longer interlock delay diminishes a switching ratio especially for high switching frequencies and causes distortion of the transmitted signal. During the time of the interlock delay the load current flows via the body diode of one of the two power switches causing additional losses due to worse switching characteristics of the body diode in comparison with the turned-on power switch. A further aspect of a long interlock delay are the charge carriers, which occur in the body diode conducting time. These charge carriers, which form the so called reverse recovery charge in the first power switch are discharged by the second power switch. Such situation is known as hard switching.
A conventional way of reducing such switching losses is by adding an additional resonant circuit allowing a soft switching. In the case that the hard switching is not eliminated the common way of reducing the switching losses is the reduction of the PWM frequency or by providing a circuit having an additional fast diode connected in parallel with the source S and drain D of the power switches PT1 and PT2, which over takes the load instead of the parasitic diode. The reduction of a working PWM frequency can be only useful for the application circuits having a sufficient inductance of the load.
An ideal hard switching with minimum power losses could theoretically be reached in the case of an interlock delay short enough (within 10 nsec range), such that the charge carriers do not form and further no reverse recovery in the hard switch situation can occur. Therefore, the switching losses would be significantly smaller.
In order to follow the trend of decreasing the weight and size of the switching conversion devices increasing the carrier frequencies obligatory. In this way a higher resolution and higher frequencies of the generated output voltage can be achieved. With the switching frequency increase the response time for rapid load changes becomes shorter and operation frequencies high enough even for the modern electro motors and audio class D amplifiers will be achieved.
The state-of-art power switch driving technologies do not reduce the interlock delay such that it becomes short enough to decrease the switching losses to a range which allows to significantly increase the switching frequencies in order to follow the later trend.